The present invention relates to an image input apparatus and an output control method, for example, in a color scanner or a color digital copying machine, which reads image information by scanning an original using a color CCD line sensor.
A 4-line CCD sensor has conventionally been marketed as a CCD line sensor used in a reduction optical system. The 4-line CCD sensor includes a sensor section wherein color filters are not arranged on a light-receiving surface of a photodiode array, and a 3-line CCD sensor wherein color filters having characteristics of the optical three primary colors of RED, GREEN and BLUE.
In order to reduce a unit device price of a CCD sensor and to compose an optical system using inexpensive parts, there is a tendency that the pixel size of the CCD sensor is reduced and the chip length is increased.
Specifically, there is a tendency that the pixel pitch (inter-pixel distance) is reduced from several-ten μm to 7.0 μm, and further to 4.7 μm.
In terms of operation speed, a higher speed is required by the system side using a CCD line sensor.
A CCD line sensor is a device for converting received optical energy to electrical energy (specifically, voltage). If the pixel pitch decreases, the angular aperture, which is a light-receiving area, decreases. As a result, an output amplitude decreases.
To solve this problem, there is a means for increasing an incident light amount. However, the amount of light of a generally used white xenon lamp is limited. It is known that if the amount of light is increased, the life decreases.
Another means for increasing the amplitude of an output signal is a method wherein the amplification factor of an output amplifier provided at the final stage of the CCD line sensor is increased. According to this method, even if the amount of charge converted by the photodiode array is small, the charge can be electrically amplified and a desired output signal amplitude can be obtained. Therefore, at present, there are many cases where the amplification factor of the amplifier at the output stage is set at a high value, and the sensor having a high sensitivity as a selling point is marketed.
Based on the above background, CCD line sensors, which meet requirements of the market, that is, a high integration density with a single package containing a plurality of photodiode arrays, a small size with a narrow pixel pitch, a high sensitivity with an increased internal amplification factor and a high operation speed, have recently been put on the market.
However, owing to the high integration density and small size, the inter-wire distance in the CCD line sensor decreases, and it becomes difficult to keep an area for a guard-ring pattern, etc. for preventing mixing of radiant noise. Furthermore, since high-speed driving is required, a large current is needed for internal control signals.
Although driving with a large current is required, protection against internal radiant noise is not provided. Thus, noise is possibly superimposed on internal wiring.
Besides, since the internal amplification factor is increased for the high sensitivity, slight induced noise may greatly be amplified at the final stage and output. In fact, there is a CCD line sensor wherein induced noise is superimposed on an output signal and an output waveform is disturbed.
As has been described above, a 3-line CCD sensor or a 4-line CCD sensor comprising a plurality of photodiode arrays has such advantages that the high-speed operation is performed and the output signal amplitude is easily attained. On the other hand, it has such a disadvantage that noise of a drive signal tends to be superimposed on an output signal waveform. If such noise is superimposed, good image information cannot be obtained.